(1) Field of the Invention
This invention relates to high-fidelity vacuum-tube audio power amplifiers for driving a loudspeaker to provide music reproduction without audible distortion, and more particularly, to an amplifier having a cathode-follower output stage requiring a large drive voltage swing provided by a novel series-connected drive stage.
(2) Brief Description of the Related Prior Art
In the past decade of the art of high-end audio reproduction it has come to be recognized by many audiophiles and amplifier designers that vacuum-tube amplifiers provide superior music reproduction as compared with transistor amplifiers. These tube amplifiers generally have output stage configurations which are either push-pull or single-ended.
Both the single-ended output stage and the push-pull output stage operate in the common-cathode mode which has numerous disadvantages, primarily due to the relatively large magnitude of the tube plate resistance. As explained in more detail below, these disadvantages include the need for a large air gap in the single-ended transformer core to prevent saturation thereby reducing the transformer primary inductance and increasing the distortion due to the resulting steep elliptical loadline traversed by the tube operating point. Further, the high source impedance, inherent in the common-cathode mode, also increases the distortion due to the nonlinear magnetization current of the transformer core. It also results in a high amplifier output impedance at the speaker terminals thereby providing a low amplifier damping factor and poor speaker transient response. A further disadvantage of the high source impedance of the output stage is the resulting frequency response nonlinearity when driving typical speakers which have an impedance which varies with frequency. In addition, the high source impedance of the plate resistance coacts with the leakage inductance and winding capacitance of the output transformer to generate phase shift and rolloff at high frequencies. The common-cathode mode also results in a high Miller-effect capacitance at the input of the output stage.
In a push-pull configuration, the high source impedance due to the plate resistance exacerbates the inevitable dynamic and static imbalances of a push-pull output stage accompanying the aging of the output tubes with use over time, and also those imbalances due to asymmetrical drive signals transmitted to the two halves of the push-pull stages.
These disadvantages of the common-cathode output stage are further explained in the detailed description of the prior art set forth below. It is well-known in the prior art that a lower output impedance and reduced distortion in the output stage itself can be obtained by utilizing a cathode-follower mode of operation for the output configuration. However, this has heretofore been impractical because the cathode-follower output stage requires a very large drive signal so that the preceding conventional drive stage would have generated as much or more distortion than would have been reduced by the cathode-follower output mode. Also, the large supply voltage required for the drive stage would have subjected the drive stage tube to an excessively high plate-to-cathode voltage.